In a communication system, a device (typically a mobile device) termed user equipment (UE) communicates wirelessly with a core network via a radio access network. Communications between the UE and the radio access network are effected in accordance with a multi-layered communications protocol.
The multi-layered communications protocol is typically represented as comprising an access stratum (AS) which includes a plurality of layers responsible for transporting data between the UE and the radio access network, and a Non-access stratum (NAS) which represents a set of higher layer protocols which use the AS to transfer data between the UE and the radio access network. The NAS is divided into two sub-layers: the Connection Management (CM) sub-layer and the Mobility Management (MM) sub-layer. The CM sub-layer includes call control protocol entities which are responsible for establishing, maintaining and terminating calls between the UE and core network.
The call control protocol is described in more detail in chapter 5.1 of the document “3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile radio interface Layer 3 specification; Core network protocols” Serial No. 3GPP TS 24.008 specification.